Conventionally, a method of manufacturing a ring-shaped permanent magnet includes the steps of filling powder of magnet molding material into a cavity of a metal die unit and molding the powder of the magnet molding material by press working while magnetically orienting the magnet molding powder by applying a magnetic field to the powder by means of a pair of coils situated in a surrounding area of the metal die unit. The metal die unit has a lower die section which includes a core, a lower punch and a die into which the core and the lower punch are inserted, creating a cavity in the lower die section. An upper punch positioned against the die and the lower punch pressurizes magnetic powder placed in the cavity.
If such a molding method is used for forming axially long cylindrical magnets in forming these radially oriented anisotropic ring magnets in a magnetic field which are widely used in small motors, there arises a problem that a sufficient degree of orienting magnetic field intensity is not obtained, resulting in a reduction in the degree of magnetic orientation of the magnetic powder and an inability to achieve high magnetic properties.
Generally, when a ring magnet is radially oriented magnetically, a magnetic flux passing through a core of a metal die unit for molding magnetic powder into a ring shape becomes equal to a magnetic flux passing within the inside perimeter of a die. Therefore, expressing the inside diameter of the ring magnet (the core diameter of the metal die unit) as Di, the outside diameter of the ring magnet (the inside diameter of the die of the metal die unit) as Do, the height of the ring magnet as H, the magnetic flux passing through the core of the metal die unit as Bc, and the magnetic flux passing within the inside perimeter of the die as Bd, there is a relationship given by equation (1) below:2xp/4xDi2xBc=pxDoxHxBd  (1)
A steel product, such as S45C, if used for the core of the metal die unit, has a saturation flux density of approximately 1.5T. In this case, substituting Bc=1.5 in equation (1) above and assuming that a magnetic field necessary for magnetic orientation is equal to or larger than 1.0T which translates to Bd=1.0T, the height H of the ring magnet which can be molded with magnetic orientation is given by equation (2) below:H=3Di2/4Do  (2)
A problem which will arise when the ring magnet is formed in a magnetic field is a reduction in magnetic orientation performance that occurs if the axial length of the ring magnet exceeds the value of H of equation (2) above. Accordingly, conventional practice has been to produce ring magnet pieces each having a short axial length which is equal to or smaller than the value of H of equation (2) above and join them with a bonding agent, for instance, to manufacture a ring magnet having a necessary axial length.
Also, as shown in Japanese Patent Application Publication No. Hei 9-233776, for example, there is proposed a method of forming a magnet having a necessary axial length by laminating molded magnet pieces in a metal die, each of the molded magnet pieces having a length that falls within a range in which these magnet pieces can be formed in a magnetic field.
Also, as shown in Japanese Patent Application Publication No. Hei 10-55914, for example, there is proposed a method including the steps of forming preliminarily molded pieces in a magnetic field and joining a plurality of these preliminarily molded pieces into a single structure by pressurizing the same with a pressurizing force which is greater than a pressurizing force applied during preliminary molding.
While a conventional radially oriented anisotropic ring magnet is manufactured by forming magnet pieces magnetically oriented in radial directions and each having a short axial length in a particular shape and bonding and stacking the magnet pieces with a bonding agent to make a ring magnet having a necessary axial length, this method is poor in productivity. Additionally, this method has a problem that the accuracy of outside diameter worsens due to misalignment of central axes of the individual magnet pieces when stacked, and unevenness of air gap occurs when the magnet pieces are assembled with a stator, for instance, resulting in a deterioration in the precision in shape at assembly as well as in magnetic properties.
Also, in the method in which a molded magnet unit having a necessarily long axial length is formed by stacking molded magnet pieces each having a short axial length in a metal die, magnetic powder is additionally filled on top of an already molded magnet piece and a newly molded magnet piece is formed thereon while applying a magnetic field. Therefore, there has been a problem that magnetic orientation in the proximity of a lamination boundary surface between the previously molded magnet piece and the newly molded magnet piece is likely to be disturbed, resulting in a deterioration of magnetic properties.
Also, there has been a problem that magnetic orientation is apt to be disturbed in the proximity of side surfaces of the molded magnet unit as the side surfaces slide along the metal die when the already molded magnet unit is moved to a position below the metal die, resulting in a deterioration of magnetic properties.
Also, since an initially molded magnet piece is subjected to repeated cycles of pressurization, there occurs a difference in the number of pressurization cycles between the initially molded magnet piece and a finally molded magnet piece, resulting in a density difference therebetween, consequently causing a problem that deformation occurs during sintering.
Also, in the method in which a plurality of preliminarily molded pieces are joined into a single structure by pressurizing the same with the pressurizing force which is greater than the pressurizing force applied during preliminary molding, there is a problem that a larger molding facility is required for repressurization and the preliminarily molded pieces are susceptible to breakage at the time of repressurization.
This invention has been made to solve the aforementioned problems. Accordingly, it is an object of the invention to provide a ring magnet and a method of manufacturing the ring magnet which make it possible to achieve a reduction in deterioration of magnetic properties due to a disturbance of magnetic orientation in the proximity of each lamination boundary surface between laminated preliminarily molded pieces, as well as high productivity and improved precision in shape.